Carbon dioxide (CO2) is a greenhouse gas believed to contribute to global climate warming. The continued, and projected expanded, use of fossil fuels throughout the world will increase the discharge of CO2. Without control of such emissions, global climate warming may prove to be disastrous to the environment. CO2 can be removed from gas streams produced by the combustion of carbon-containing compounds. Methods for the removal of CO2 from gas streams include chemical absorption/adsorption with a solvent, membrane separation, cryogenic fractionation, and adsorption using molecular sieves. Each of these processes are expensive and may be cost-prohibitive for a particular application.
Another method for the removal of CO2 from a gas stream is dry scrubbing or chemical absorption/adsorption with a sorbent. A dry, regenerable sorbent process for the removal of CO2 from a gas stream is described here. The sorbent used is an alkali-metal compound, alkaline-earth metal compound, or amine/amide compound by itself or deposited on a substrate. The sorbent is used to remove CO2 from gas that can be produced by the combustion of coal, oil or natural gas and is then regenerated and recycled to repeat further CO2 removal. The process is continuous and uses a moving-bed reactor in a cross-flow arrangement. Disposal of the recovered, concentrated carbon dioxide stream is a separate issue from what is described here.
Dry scrubbing processes are so named because the resulting absorptive/adsorptive material (xe2x80x9cactive materialxe2x80x9d) being used is a solid. The dry scrubbing process described here can use dry alkali-metal compounds or dry alkaline-earth metal compounds. Wet scrubbing processes use a liquid active material and typically use an aqueous slurry as the active material. Wet scrubbing processes are most commonly known and used to remove sulfur dioxide (SO2) from stack gases emitted by steam generation plants.
Wet scrubbing technologies require greater volume for processing than do dry processes. For example, if a wet scrubbing system were retrofitted onto an existing power plant, the land area required for the wet scrubber would be almost as vast as the power plant itself. With the elimination of large quantities of liquid required for a wet scrubbing process, the land area required will be considerably less, and the overall energy consumption required to operate the system will be reduced. The contact of CO2-laden gas with a solid bed of active material (rather than with a wet active material) also allows for better temperature control. Therefore, the dry scrubbing process described here is economically advantageous over commercially available wet scrubbing technologies.
Both wet and dry scrubbing processes may be either disposable or regenerative systems. Regenerative systems are designed to further process the active material, making it suitable for subsequent productive passes through the scrubber. In disposable systems, the active material will make only one pass through the scrubber and will then be discarded. Disposable systems are less desirable due to the added expense and maintenance involved with the disposal of larger amounts of spent active material.
Both cross-flow and countercurrent flow arrangements are known and used in gas scrubbing processes. With a cross-flow arrangement, the active material travels from the top of the reactor to the bottom while the gas to be treated flows horizontally through the active material. In countercurrent flow systems, the active material moves from the top of the reactor toward the bottom while the gas enters the reactor at the bottom and flows upward and out the top. Similarly, moving-bed reactors and fluidized bed reactors are known and used in gas treatment processes. A cross-flow moving-bed reactor consists of a packed bed of solids that is fixed together and moves through the reactor. This panel of solids may be beads or pellets of sorbent that are stacked one on top of the other and held in place by retention screens. The flow of solids may occur very slowly within the moving-bed reactor. Whereas a fluidized bed is a highly mixed bed of material. Gas flows upward through the bed of solids with sufficient velocity to lift the bed out of a stationary position. A gas distributor is usually used within a fluidized bed to prevent the material from falling out of the bottom of the vessel.
Moving-bed reactors for the treatment of flue gas have been described (see U.S. Pat. No. 5,169,607 issued to Krambrock, et. al. (xe2x80x9cKrambrockxe2x80x9d)). However, the reactor described by Krambrock uses a countercurrent flow of loose material and flue gas. Krambrock stated several reasons for not using a cross-flow arrangement. One of these reasons was the shorter retention time with cross-flow as compared to a countercurrent flow arrangement. Much of the research done on flue gas cleanup has focused on the removal of sulfur dioxide or nitrogen-containing contaminants (SO2 or NOx). The process described here works well with cross-flow arrangement because CO2 ab/adsorption chemistry is sufficiently fast so as not to require long gas residence time.
Krambrock""s focus was on providing uniform coverage of the loose material inside the reactor. No specific process for removal of waste gas constituents was described. The process described here uses a cross-flow arrangement of treatment material relative to gas flow. The process described here also focuses on the removal of CO2 from the gas stream and specifically describes the process by which such removal may be accomplished and lists several appropriate sorbents.
Processes for the adsorption/desorption of CO2 have been described before (see U.S. Pat. No. 4,937,059 issued to Kolts, et. al. (xe2x80x9cKoltsxe2x80x9d)). The process described by Kolts was not continuous as Kolts describes a swing setup for absorption/desorption, and the sorbents used were alkali-metal compounds that were required to contain lanthanum oxide (La2O3). A continuous process is described here and, if the sorbents used are alkali-metal compounds, there is no requirement that the active material contain La2O3.
The process described here can be installed at new facilities or can be retrofitted into an existing producer of CO2-containing gases, e.g. a utility or industrial boiler. Depending upon the optimum temperature of CO2 absorption with the sorbent, the absorber could be placed anywhere along the gas stream that may have been or will be treated with another scrubbing process to remove other pollutants. The process may be installed anywhere along the gas path of new advanced power systems such as Integrated Gasification Combined Cycle (IGCC), Low-Emissions Boiler Systems (LEBS), High Performance Power Systems (HIPPS), and Pressurized Fluid Bed Combustors (PFB). In addition, the process may be utilized with any system that produces CO2 either as a product, by-product, waste, or in any system where the objective is to concentrate CO2.
The primary objective of this invention is to provide a continuous ab/adsorption process for removing carbon dioxide from a gas stream with the active material being an alkali-metal compound or alkaline-earth metal compound.
Another objective of this invention is to provide a process for regenerating and reusing the active material used to remove carbon dioxide from the gas stream.
A carbon dioxide-laden gas stream enters a moving-bed reactor and contacts an active material that is an alkali-metal compound or alkaline-earth metal compound by itself or supported on a substrate of carbon, alumina, silica, titania or aluminosilicate. The carbon dioxide reacts with the metal compound to generate bicarbonate when the active material is an alkali-metal compound or carbonate when the active material is an alkaline-earth metal compound.
The spent sorbent containing the bicarbonate or carbonate is moved through a second reactor where it is heated or treated with a reducing agent to release concentrated carbon dioxide gas and thereby regenerating the active material for reuse as the sorbent material in the first reactor. New, make up sorbent may be added to the regenerated sorbent prior to subsequent passes in the carbon dioxide removal reactor.